In the electrical arts, it is frequently desirable to determine the value of an unknown capacitor. Several approaches are known for making such a determination as, for example, that disclosed in U.S. Pat. No. 3,824,459 which describes an apparatus for finding the value of a capacitance by generating counted pulses representative thereof. The number of these generated pulses is dependent upon the accuracy of the determined values of a pair of resistors. Yet another example is shown in U.S. Pat. No. 4,065,715 which describes a circuit for simultaneously charging a reference capacitor of known value and a second capacitor of unknown value. Each capacitor is connected in parallel with its own resistor and the voltage across each capacitor is directed to a separate threshold detector. The value of the unknown capacitance is determinable by measuring the difference in time between that required to trigger the first detector and that required to trigger the second detector.
Yet another type of capacitance transducer is shown in U.S. Pat. No. 4,227,419. The circuit disclosed therein uses a common current source to charge either of two capacitors and provides an output pulse train having positive and negative portions, the durations of which are compared for determining the value of a varying capacitance with respect to a known, fixed capacitance. The apparatus uses two comparator-detector circuits for triggering a flip-flop circuit. Another circuit for measuring capacitance is shown and described in a bulletin titled "Capacitive Humidity Sensor, Technical Information 063", believed to have been published by U.S. Philips Corp. The circuit disclosed therein uses a free running multivibrator with a second multivibrator slaved thereto. These timing circuits generate pulses useful for capacitance determination. Each multivibrator uses a separate but nominally equal trigger voltage level and has a characteristic cyclic period which is dependent upon the value of a resistor dedicated to a particular multivibrator.
While these known devices and methods have heretofore been generally satistactory, they tend to be characterized by certain disadvantages. In particular, they variously employ separate comparator circuits for triggering purposes, separate trigger voltage reference sources and/or separate resistive elements for capacitance determination. It is known to those in the electrical arts that certain characteristics of the surrounding ambient, most notably temperature, may affect the values of these components with a resultant loss in measurement accuracy. An apparatus and method useful in making capacitance measurements which employs a single, common resistive element for capacitor charging and a single voltage reference for generating a trigger or clocking pulse would be a significant advance in the art. The inventive apparatus and method are seen as being particularly useful in heating, ventilating and air conditioning (HVAC) applications where the value of the measured capacitance is representative of relative humidity, pressure, temperature or other parameter.
It is an object of the invention to provide an apparatus and method for capacitance measurement which overcomes the disadvantages of the prior art.
Another object of the present invention is to provide a capacitance measuring apparatus which utilizes a single, common resistive element for capacitor charging.
Yet another object of the invention is to provide an apparatus and method for capacitance measurement which utilizes a single, common trigger reference for generating clocking pulses useful in making such measurements.
Still another object of the present invention is to provide an apparatus and method for determining the value of a capacitor wherein the effect of changes in ambient temperature is reduced.
Another object of the present invention is to provide an apparatus and method for capacitance measurement which makes advantageous use of integrated circuitry. These and other objects of the invention will become more apparent from the detailed description thereof taken in conjunction with the accompaning drawing.